The present invention, in some embodiments thereof, relates to semiconductors, and more particularly, but not exclusively, to high-indium content quantum wells having controllable nitrogen content, processes of preparing same by atomic layer epitaxy, superlattices containing same and uses thereof.
Conventional double heterostructure (DH) junctions are generally fabricated from the GaAs—(In/Al)GaAs materials system and include a pair of wide bandgap layers, such as (In/Al)GaAs, of opposite conductivity type and an active region, such as GaAs, sandwiched therebetween. The interfaces between the active region and the wide bandgap layers form a pair of heterojunctions which provide both optical and charge carrier confinement. From the standpoint of quantum effects, when the active region of a DH junction is thicker than 500 Angstroms, the discrete energy levels associated with confined electrons are so closely spaced that quantum effects are negligible. When the passive layers are thin enough (less than about 500 Angstroms), the carriers are able to distribute themselves among the active layers, and when the active layers are thin enough (less than about 500 Angstroms), the confined carriers are characterized by discrete energy levels.
A quantum well is therefore a double heterostructure potential well that confines particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the wavelength of the charge carriers, namely electrons and holes, leading to energy levels called “energy subbands”, wherein the energy of the carriers has discrete values. Quantum wells structures are generally grown by molecular beam epitaxy or chemical vapor deposition with control of the layer thickness down to monolayers, as disclosed originally in U.S. Pat. No. 3,982,207.
In recent years, the GaInAs dilute nitride alloy has been the subject of intense theoretical and experimental research due to its unique physical properties, making it highly suitable for forming quantum wells and other structures. The possibility of varying the lattice constant and band-gap energy of GaInAs nitride alloy (GaInAsN) over a wide range by optimizing the nitrogen content [Henini, M., Dilute Nitride Semiconductors, Elsevier, Oxford, 2005] provides an opportunity to tailor the material's properties for a desired optoelectronic device application, such as near-infrared (IR) lasers emitting in the optical fiber communication wavelength window [see, for example, U.S. Pat. Nos. 7,109,526 7,256,417 and 7,457,338 and U.S. Patent Application Publication Nos. 2005-0056868 2005-0173694 and 2007-0248135], and quantum-well IR photodetector (QWIP) devices with wavelengths shorter than 4 μm [Albo, A. et al., App. Phys. Lett., 94, 093503, 2009].
However, the growth of high-indium-content dilute-nitride GaInAsN/GaAs structures using metal organic chemical vapor deposition (MOCVD) is still a challenging industrial process. One of the challenges in growing GaInAsN quantum-well (QW) lasers using MOCVD is the difficulty of incorporating nitrogen atoms into GaInAs quantum wells. The rate of incorporation of nitrogen is markedly reduced in high-indium-content quantum wells due to the weak In—N bonds which cannot compete with the rate of desorption of nitrogen from the surface during growth [Saito, H. et al., J. of Crystal Growth, 195, 416, 1998], and low growth temperatures combined with a high dimethylhydrazine (DMH or DMHy) molar fraction ratio are needed to incorporate a reasonable amount of nitrogen. On the other hand, growth at low temperatures results in a poor crystal quality [Hakkarainen, T. et al., J. Phys: Condens. Matter., 16, S3009, 2004].
Atomic layer epitaxy (ALE) is a chemical process by which conformal thin-films of materials (precursors) are deposited sequentially onto various substrates. ALE is unique and different from conventional chemical vapor deposition (CVD), molecular beam or hot-wall epitaxy methods, mainly in the order and manner in which the different substances are introduced into the system. In ALE the substances are introduced individually, one at a time (cycle) and the system is purged between cycles. The method is also different then other deposition methods by being self-limiting, namely the amount of the film material deposited in each reaction cycle is constant rather than being dependent on precursor concentration and exposure time. Thus, the ALE reaction splits the CVD reaction into two half-reactions or more, keeping each of the precursor materials temporally separate during the reaction. Due to the characteristics of self-limiting and surface reactions, ALE film growth makes atomic scale deposition highly controllable. Separation of the precursors is accomplished by pulsing a purge gas (typically nitrogen or argon) after each precursor pulse to remove excess precursor from the process chamber and prevent ‘parasitic’ CVD events occurring on the substrate. By keeping the precursors separate throughout the coating process, atomic layer control of film growth can be maximized, affording conformal films of uniform thickness.
ALE can be used to deposit several types of thin films, including elemental metals, metal oxides, metal nitrides and metal sulfides. The growth of material layers by ALE consists of preparing the surface, typically by heat, and repeating a cycle of four basic steps starting with exposure of the first precursor; purge or evacuation of the reaction chamber to remove the non-reacted precursors and the gaseous reaction by-products; exposure of the second precursor or another treatment to activate the surface again for the reaction of the first precursor; and purge or evacuation of the reaction chamber. Each monolayer growth cycle adds a given amount of material to the surface, referred to as the growth per cycle. To grow a material layer, reaction cycles are repeated as many as required for the desired film thickness, while a growth cycle may span from half a second to a few seconds and deposit between 0.1 and 3 Å of film thickness.
In contrast to QW prepared by conventional MOCVD, high-quality layers growth of InAs, GaAs and GaInAs can be obtained at relatively low temperatures ALE using group elements [Tischler, M. A. et al., Appl. Phys. Lett., 49, 1199, 1986 and Bedair, S. M. et al., J. Sci. Technol. B, 12, 179, 1994]. During the ALE process, the III and V atomic sources are alternately and separately introduced into the reactor. This process enables monolayer growth per cycle without the elemental sources interacting, while in conventional growth, all the elements arrive at the substrate simultaneously.
Ishida, A. et al. reported a series of AlN/GaN short-period superlattices with AlN monolayer, prepared by hot-wall epitaxy [Physica E: Low-dimensional Systems and Nanostructures, 13, 2-4, pp. 1098-1101, 2002]. Janga, Y. D. et al. report InAsN quantum dots (QD) on GaAs with an intense and narrow photoluminescence peak at 1.3 μm [Janga, Y. D. et al., Physica E, 17, pp. 127-128, 2003]. Yoon, S. F. et al. report InNAs and GaInNAs self-assembled quantum dots and lasers grown by solid source molecular beam epitaxy [Yoon, S. F. et al., Nanoscale Research Letters, 1, 1, pp 20-31, 2006]. Kuroda, M. et al. report of growth and characterization of InAsN alloy films and quantum wells [Kuroda, M. et al., Journal of Crystal Growth, 278, 1-4, pp. 254-258, 2005]. Gein, C. H. et al., suggested the superlattice structure approach and disclosed theoretical performance of InAs/InGaSb superlattice-based midwave infrared lasers [J. Appl. Phys., 76, pp 1940-1942, 1994]. Hasenberg, T. C. et al., demonstrated a 3.5 μm GaInSb/InAs superlattice diode laser [Electronics Letters, 31, 275-276, 1995]. Khandekar, A. A. et al., reported a type II superlattice dilute-nitride laser structure and characteristics of GaAsN/GaAsSb type-II quantum wells grown by metalorganic vapor phase epitaxy on GaAs substrates [J. Appl. Phys., 98, 12, p. 123525, 2005]. Additional background art can be found, for example, in U.S. Pat. Nos. 5,155,571, 7,391,507 and U.S. Patent Application Publication No. 2010-0118905.